Domestic Hot Water not reaching temp

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Vokera Linea Plus AG Combi

For years the DHW thermostat on the boiler was set for a water temp of 50C (on the boiler display) when a tap or our Mira Excel shower valve was fully open. This temp can no longer be achieved, summer or winter, regardless of thermostat setting. Typically, right now, it will get to a max of around 41C. Higher temp can only be achieved by restricting the flow at a tap (restricting the shower valve doesn't seem to have so much effect unless severely gagged to an almost unusable flow).
Boiler serviced annually, CH seems side ok, softish water area, whole system chemically cleaned this year and filter fitted, then heat exchanger removed, visually checked and flushed - all made no difference.
Two heating companies don't know what's wrong.
Hope this gives enough info and makes sense.
Any thoughts or ideas?
Many thanks in advance!
 
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Try isolating the shower - it might be passing cold water through the valve into the hot supply, cooling it down
 
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Easier to turn off cold feed into boiler and see if any water comes out of the hot taps!

I do wonder when the boiler was last properly serviced.

There are gas settings which can drift in not checked and reset and can cause issues like yours.

Tony
 
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Try isolating the shower - it might be passing cold water through the valve into the hot supply, cooling it down
Thanks muggles. Unfortunately there are no isolation valves on anything in the house (except boiler and toilet). I suppose any mixer tap could be passing cold into the hot? All taps are affected and only get properly hot now if opened about half way.
 
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Easier to turn off cold feed into boiler and see if any water comes out of the hot taps!

I do wonder when the boiler was last properly serviced.

There are gas settings which can drift in not checked and reset and can cause issues like yours.

Tony
Thanks Agile. Certainly been serviced every year, whether properly I cant tell! If cold feed to boiler is turned of and water comes out of taps, what does that suggest? Ta
 
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It demonstrates that cold is leaking into the hot!

When you do it also move any joystick mixer taps and see if there are any positions when water comes out fropm the hot taps.

Who did services and how long did it take? Did case come off boiler?

Tony
 
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It demonstrates that cold is leaking into the hot!

When you do it also move any joystick mixer taps and see if there are any positions when water comes out fropm the hot taps.

Who did services and how long did it take? Did case come off boiler?

Tony
Turned off cold feed into boiler and no water came out of the hot taps or any of the mixer taps when in the hot position. Always serviced by Corgi or GSR engineers. Most recent company has been established since 1926. From memory at last service, outer case off, burner area covers off, brushed down, gas pressure tested, burner pressures tested, emissions checked, expansion pressure checked. Took about 25-30 mins I think. Hope that covers all questions. Ta.
 
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Update. As of this morning the DHW only got to 35C and 3 downstairs rads nearest manifold got warm (boiler is upstairs) so I'm guessing that shouldn't happen and does indicate diverter problem?
 

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